This invention relates in general to means for fastening structures together in an offset or spaced relationship and, more specifically, to fasteners which resist the transfer of thermal stresses and the transfer of heat from one structure to the other.
Many difficulties have been encountered, particularly in aerospace applications, in attempting to design fasteners which are free of the thermal stress which results from differential thermal expansion between dissimilar fastener and sheet materials. For example, the coefficient of thermal expansion (CTE) of carbon-carbon materials is approximately an order of magnitude lower than the CTE of typical fastener materials. This thermal expansion mismatch can cause failure of the carbon-carbon material around a standard, snug-fitting, cylindrical fastener. A clearance left around the fastener to accommodate the fastener expansion upon heating can make the joint unacceptably loose at low temperatures.
Various conical or biconic fasteners have been developed in an attempt to overcome this problem. Typically are the daze fastener system disclosed by Jackson et al. in U.S. Pat. No. 4,512,699, the various biconic fasteners and rotating bearings disclosed by Kleven in U.S. Pat. No. 3,107,950 and the various biconic and curved surface fasteners described by Blosser et al. in their paper "Theoretical Basis for Design of Thermal-stressfree Fasteners", NASA Technical Paper 2226, December 1983.
Each of these utilize a bolt-type fastener for fastening two plates of dissimilar material in a face-to-face, pressed together, arrangement. Basically, these bolts consist of a through-bolt having one or a pair of conical washers in conical recesses in the outer plate surface, so as the through-bolt is tightened, the conical washer surfaces are pressed against the recesses, tightening the plates together. In most cases, the vertices of the conical washers are coincident, substantially lying in the plane of contact of the two plates.
Since the patentees and authors of the cited documents recognize that this basic biconical fastener does not entirely eliminate the differential CTE problem, especially where the material CTE's are not isotropic, they describe a large number of variations and alternatives in an attempt to meet specific problems. Typical of these are the concave and convex surfaces of Blosser et al., the enlarged, hollow and malleable conical heads of Jacobson et al. and the intermediate ball bearings and roller bearings proposed by Kleven.
Each of the fastening means described by these patentees and authors is only useful for fastening two materials together in an abutting relationship. In many cases, it would be necessary or desirable to fasten structural panels or other structures to supports in an offset and spaced relationship.
Thus, there is a continuing need for improved fasteners for securely connecting two or more structures in a spaced relationship while eliminating any damaging stresses due to differences in CTE and limiting the heat flow between the structures.